I definitely identify with many of the opinions in here - I'll mute the music in some games as well if I'm interested in listening to something else, or if I'll be playing with friends on voice chat. I also understand that since this won't be a single-player game, the amount the music will affect the experience is less than it might have been otherwise.
That being said, I'm still of the mindset that if it's worth adding music at all, it's probably worth doing right. And there are some games that really got the whole music thing down - I imagine most (or maybe even all) of us would recognize the Halo theme with very little prompting, for instance.
I've been thinking about how music might be done to make it more dynamic without putting too much extra work into it. As I recall (unfortunately I can't check, but I'm sure someone will verify), in Sins of a Solar Empire, the music would change depending on your zoom level and how intense the fighting was at the location you were viewing. If that's not the case, I still like the idea. Anyway, for a game like this, having the music react to your own surroundings could enhance the moods of the players involved. You could have a fairly steady bass line and limited percussion on a track that essentially runs constantly, and have perhaps half a dozen other tracks that would play in various instances, sometimes simultaneously. If there's a hero around, track two would start playing with the bass track. Another hero from each side shows up, and track three boots up to bring even more intensity. Entering the enemy base might change the nature of the music, turning off tracks two and three and instead playing track five, and maybe even another track to augment how the bass line and percussion is playing out. Playing around with how the tracks enter could make those transitions work more and more smoothly. It's a lot of work, and admittedly something of a raw idea, but I think it'd be cool.
The discussion here raises another thought in my mind, too. With ambient noise (and often music), I often notice varying levels of repetition, and that sometimes puts a damper on my mood. Would you guys consider occasionally updating sound packs? Even if it's just releasing an extra few sounds here and there, the game will feel more engaging if my hero doesn't spit out the same three lines twelve times a minute. It works for LoL because of the exaggerated cartoony feel is coupled with recognizable catch phrases, but since this game aims for a different feel, that might be one way to do it.
And yes, Ian - that track does seem to fit fantasy as well as the Vasari